core: effectify CLI account commands with tagged PollResult union

- Rewrite all account CLI commands (login, logout, switch, orgs) as
  Effect.fn with AccountService accessed directly via yield*
- Convert PollResult from plain object union to Schema.TaggedClass
  variants with Schema.Union and Match.valueTags for exhaustive matching
- Add recursive poll function for device auth flow (stack-safe via
  Effect trampolining)
- Add shared Effect runtime at src/effect/runtime.ts
- Add Effect wrappers for @clack/prompts at src/cli/effect/prompt.ts
- Add @effect/language-service TS plugin for editor support
- Refactor repo tests to use testEffect helper with Layer-based setup
- Parallelize org fetching in orgs command

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>